The present invention relates to a device for interrupting high voltage currents and in particular to an improved high voltage switch that has a substantially greater voltage interrupting capability than other switches of comparable size.
It has long been recognized in high voltage applications that arc plasma is generated between points of transmission during a switching operation. The arc plasma, of course, is created by the continued transmission that occurs across a gap between two contacts due to the high voltage potential present. As long as the resistance between the two contacts remains below the conductivity ceiling necessary to support the arc plasma, a current will continue to be conducted across the contacts even though they are physically separated.
Original attempts to interrupt high voltage currents sought to minimize the effect of the arc plasma by increasing the dielectric strength of the medium between the two contacts. Thus, the liquid switch was developed which submerged the contacts in a fluid, such as oil, to aid in cooling the arc plasma so as to limit the intensity of the arc generated. The immersion of the contacts in oil also aided in controlling the problem of abnormal arcing which often occurred between alternative points exposed to electrical stress during the switching operation.
Additional efforts to combat the problem included attempts to extinguish rather than to control the arc plasma generated. These devices sought to interrupt current flow by such means as injecting a pressurized jet of fluid at the arc plasma upon separation of the energized contacts to effect extinction thereof. However, the effectiveness of such devices in handling high voltage currents was dubious at best.
In fact, since the advent of the oil switch, little has changed in the way of new technology to handle the substantially higher voltage potentials encountered in today's power systems. Essentially, the increased capacity requirements have been satisfied simply by making the size of the switches larger. Specifically, the movement of one contact relative to the other has been increased to increase the dielectric strength between the two contacts. Since it has always been assumed that the arc plasma takes the shortest possible path exposed to electrical stress, the increased contact movement has always followed a substantially linear path. Thus, the overall dimensions of the switch were enlarged to accommodate the greater distance between the two stationary switching positions.
It is this basic misconception of the prior art from which the present invention makes a dramatic departure. Although it is true that arc plasma will follow the path of least resistance, it does not necessarily follow that the path of least resistance corresponds to the shortest distance between the two energized contacts. Rather, the intense heat of the arc plasma generates a gas bubble which assumes a column-like form about the arc plasma as the length of the arc is extended. Additionally, the dielectric strength within the gas column is substantially less than the dielectric strength of the surrounding fluid insulating medium. Thus, as long as the gas column can be controlled, the path of the arc plasma can also be controlled.
Thus, it is the primary object of the present invention to provide a device for interrupting high voltage electrical currents by controlling the arc plasma generated so that it substantially follows an extended predetermined path within a substantially reduced volume as compared to devices having comparable voltage capacities.
In addition, the present invention provides a high voltage electrical switch that is inexpensive to manufacture and is readily assembled without requiring any tools or special hardware.
Furthermore, as will be appreciated by those skilled in the art, the present invention provides a high voltage electrical switch that improves the reliability of the switching operation and yet is simple to operate.
Moreover, it will be seen that although the preferred embodiment discloses a single-pole device, the design is readily adaptable to a multiple-pole arrangement.
In general, the preferred embodiment comprises a rotary type switch having its contact elements immersed in a fluid insulating medium. The switch is preferably adapted to rotate its movable contact approximately three quarters of a revolution upon rotation of the operator's handle about a quarter of a revolution. The total rotation of the operator's handle is preferably limited for ease of operation. Specifically, since high voltage switches of this type are frequently operated with a "hot stick", it will be appreciated that it becomes increasingly difficult to rotate the operator's handle over a distance much greater than 90.degree..
In addition, the preferred embodiment of the switch is designed to perform the entire switching operation upon the simple rotation of the operator's handle. The rotational energy from the operator's handle is stored in a motor spring which rapidly releases its stored rotational energy when unlatched by a cam. The cam is adapted to rotate with the operator's handle to initially store rotational energy in the motor spring and subsequently to release the rotational energy in the motor spring during the last few degrees of rotation. The rotational energy from the motor spring is applied to the input of a mechanical overdrive unit which is designed to provide approximately three times the amount of rotation at its output as is applied to its input. The output from the overdrive unit is directly coupled to the movable contact element of the switch. Thus, it can be seen that in response to the input rotation of 85.degree., the switch is designed to rotate the movable contact approximately 255.degree..
When the movable contact reaches its extent of travel, an oppositely sprung motor spring is automatically latched. To return the switch to its original closed position, the operator's handle is simply rotated in the opposite direction causing rotational energy to be stored in the opposite motor spring until the energy is released by the cam as previously discussed. The switch provides means for rapidly returning the movable contact to its original position to minimize arcing between the contacts when the switch is closed. Thus, as will subsequently be explained in greater detail, the present invention discloses a device which performs the storing, tripping, driving, and latching functions all in a single motion of the operator's handle.
In addition, an anti-stick or anti-weld feature is incorporated into the design of the device to initiate contact movement in the event that the motor spring is unable to move the contact when freed by the cam. More specifically, if the motor spring does not immediately release its stored rotational energy when unlatched, the cam is further designed to transmit the next few degrees of operator handle movement directly to the input of the overdrive unit. This results in contact movement of approximately three times the additional handle movement, which is sufficient to initiate normal switching operation.
The present invention also discloses a novel switching element that includes a unique contact design providing multiple contact paths and an "arc horn" which aids in the control of the arc plasma as well as providing a sacrificial segment in the event of a fault-make switching condition.
Finally, it will be seen from the following detailed description, that the preferred embodiment of the high voltage switch according to the present invention is readily assembled without the use of special tools, and includes means for preventing the accidental disassembly of the switch during operation thereof.